The Federated Tumor Segmentation (FeTS) challenge 2021 is the first challenge in the federated medical imaging area. The challenge data set is composed of multi-institutional magnetic resonance images from the International Brain Tumor Segmentation (BraTS) challenge and other independent institutions in the FeTS initiative [1]}, [2]}, [3]}, [4]}, [5]}. The training set contains 341 images, institution-based split of which is given in Fig. REF . The validation and the tests sets contain 111 and 166 images, respectively. The segmentation annotations of the challenge dataset were performed by annotators whose experience levels vary with respect to their clinical and academic backgrounds. Then, these annotations were approved by two experienced board-certified neuroradiologists with more than 12 years of experience [4]}.
